The ultrastructure of the excitatory (glutamate-transmitter) and inhibitory (GABA-transmitter) synapses of the neuromuscular system of the crayfish, as well as the excitatory neuromuscular junctions (acethylcholine-transmitter) of the frog, is studied. The influence of the osmolarity of the fixatives and washing solutions on the structure of the synaptic vesicles is investigated. Secondly, the fine structure of the crustacean stretch receptor neuron is studied. Emphasis is placed on the fine structural changes which occur during the stretched condition of this receptor. Thirdly, the transmission mechanism of the neuromuscular junctions of the frog is studied electrophysiologically and electron microscopically. Major methods used in these studies are electron microscopy and micro-electrophysiology.